The sky is not falling. A 12,500-pound NASA satellite the size of a school bus is, however.

It’s the Upper Atmosphere Research Satellite — or UARS — and it’s currently tumbling in orbit and succumbing to Earth’s gravity. It will crash to the surface Friday.

Or maybe Thursday. Or Saturday.

Out-of-control crashing satellites don’t lend themselves to exact estimates, even for the precision-minded folks at NASA. The uncertainty about the “when” makes the “where” all the trickier, because a small change in the timing of the re-entry translates into thousands of miles of difference in the crash site.

As of the moment, NASA says the 35-foot-long satellite will crash somewhere between 57 degrees north latitude and 57 degrees south latitude – a projected crash zone that covers most of the planet, and particularly the inhabited parts. In this hemisphere, that includes everyone living between northern Newfoundland and the frigid ocean beyond the last point of land in South America.

Polar bears and Antarctic scientists are safe.

It’s the biggest piece of NASA space junk to fall to Earth in more than 30 years. The satellite will partially burn up during re-entry and break into about 100 pieces.